The autophagic tumor stroma model of cancer or "battery-operated tumor growth": A simple solution to the autophagy paradox

Caveolin-1 and mitochondrial alterations in regenerating rat liver

The liver has a remarkable ability to regenerate after partial hepatectomy (PH), although the factors governing such ability are still poorly understood. During the prereplicative phase of the regeneration, ultrastructural alterations of periportal hepatocytes were seen, including mitochondrial swelling, abnormal accumulation of lipids, and myelin figures which could lead to the formation of lipid droplets. As it has been hypothesized that caveolin-1 is involved in lipidogenesis and in mitochondrial homeostasis, we aimed to study the subcellular distribution of caveolin-1 in hepatocytes at an early stage following PH. Liver samples were processed for light and electron microscopy at 0 h, 24 h, and 96 h after PH. The expression and subcellular distribution of caveolin-1 was assessed by immunohistochemical and immunocytochemical techniques. Following PH, at 24 h, membranes of altered mitochondria of periportal hepatocytes exhibited significant decrease of caveolin-1 expression compared with control. Myelin figures showing high expression of caveolin-1 were also seen. At 96 h, hepatocytes became ultrastructurally similar to the control liver, and the expression of caveolin-1 on mitochondria showed a moderate increase compared with 24 h after PH. Decrease of expression of caveolin-1 in the altered liver mitochondrial membranes at 24 h following PH, and the high expression of caveolin-1 observed on myelin figures, suggests involvement of caveolin-1 is in both mitochondrial homeostasis and lipidogenesis. Addressing the role played by caveolin-1 during liver regeneration might disclose additional features of mitochondrial homeostasis and lipidogenesis during frequent metabolic liver diseases.

Using the "reverse Warburg effect" to identify high-risk breast cancer patients: stromal MCT4 predicts poor clinical outcome in triple-negative breast cancers

We have recently proposed a new model of cancer metabolism to explain the role of aerobic glycolysis and L-lactate production in fueling tumor growth and metastasis. In this model, cancer cells secrete hydrogen peroxide (H2O2), initiating oxidative stress and aerobic glycolysis in the tumor stroma. This, in turn, drives L-lactate secretion from cancer-associated fibroblasts. Secreted L-lactate then fuels oxidative mitochondrial metabolism (OXPHOS) in epithelial cancer cells, by acting as a paracrine onco-metabolite. We have previously termed this type of two-compartment tumor metabolism the "Reverse Warburg Effect," as aerobic glycolysis takes place in stromal fibroblasts, rather than epithelial cancer cells. Here, we used MCT4 immuno-staining of human breast cancer tissue microarrays (TMAs; > 180 triple-negative patients) to directly assess the prognostic value of the "Reverse Warburg Effect." MCT4 expression is a functional marker of hypoxia, oxidative stress, aerobic glycolysis, and L-lactate efflux. Remarkably, high stromal MCT4 levels (score = 2) were specifically associated with decreased overall survival (< 18% survival at 10 y post-diagnosis). In contrast, patients with absent stromal MCT4 expression (score = 0), had 10-y survival rates of ~97% (p-value < 10 (-32) ). High stromal levels of MCT4 were strictly correlated with a loss of stromal Cav-1 (p-value < 10 (-14) ), a known marker of early tumor recurrence and metastasis. In fact, the combined use of stromal Cav-1 and stromal MCT4 allowed us to more precisely identify high-risk triple-negative breast cancer patients, consistent with the goal of individualized risk-assessment and personalized cancer treatment. However, epithelial MCT4 staining had no prognostic value, indicating that the "conventional" Warburg effect does not predict clinical outcome. Thus, the "Reverse Warburg Effect" or "parasitic" energy-transfer is a key determinant of poor overall patient survival. As MCT4 is a druggable-target, MCT4 inhibitors should be developed for the treatment of aggressive breast cancers, and possibly other types of human cancers. Similarly, we discuss how stromal MCT4 could be used as a biomarker for identifying high-risk cancer patients that could likely benefit from treatment with FDA-approved drugs or existing MCT-inhibitors (such as, AR-C155858, AR-C117977, and AZD-3965).

MicroRNA: a third dimension in autophagy

Autophagy is a catabolic process that allows cellular macromolecules to be broken down and recycled into metabolic precursors. It is a highly conserved, critical process, allowing cells to gain survival advantages under various stress situations due to growth and environmental changes. In the past few years, mounting evidence indicates that the post-transcriptional and translational controls mediated by non-coding miRNAs contribute significantly to autophagy in cancer. Such acute modulation of protein synthesis mediated by miRNAs provides cells with advantages in response to starvation, genotoxic stress and hypoxia. In this review, we highlight some of the important discoveries and molecular insights of miRNAs in regulating autophagy based on various cancer models.

Is cancer a metabolic rebellion against host aging? In the quest for immortality, tumor cells try to save themselves by boosting mitochondrial metabolism

Aging drives large systemic reductions in oxidative mitochondrial function, shifting the entire body metabolically towards aerobic glycolysis, a.k.a, the Warburg effect. Aging is also one of the most significant risk factors for the development of human cancers, including breast tumors. How are these two findings connected? One simplistic idea is that cancer cells rebel against the aging process by increasing their capacity for oxidative mitochondrial metabolism (OXPHOS). Then, local and systemic aerobic glycolysis in the aging host would provide energy-rich mitochondrial fuels (such as L-lactate and ketones) to directly "fuel" tumor cell growth and metastasis. This would establish a type of parasite-host relationship or "two-compartment tumor metabolism", with glycolytic/oxidative metabolic-coupling. The cancer cells ("the seeds") would flourish in this nutrient-rich microenvironment ("the soil"), which has been fertilized by host aging. In this scenario, cancer cells are only trying to save themselves from the consequences of aging, by engineering a metabolic mutiny, through the amplification of mitochondrial metabolism. We discuss the recent findings of Drs. Ron DePinho (MD Anderson) and Craig Thomspson (Sloan-Kettering) that are also consistent with this new hypothesis, linking cancer progression with metabolic aging. Using data mining and bioinformatics approaches, we also provide key evidence of a role for PGC1a/NRF1 signaling in the pathogenesis of (1) two-compartment tumor metabolism, and (2) mitochondrial biogenesis in human breast cancer cells.

Caveolin-1 and breast cancer: a new clinical perspective

The current chapter focuses on the role of Caveolin-1 (Cav-1) in cellular growth with an emphasis on its implication in breast cancer initiation, progression, clinical prognosis and as a potential therapeutic target. The role of Cav-1 as a tumor suppressor in breast cancer has emerged in the past few years, with dual functions on both cancer epithelium as well as the cancer stroma. Its multiple functions as a regulator of estrogen signaling and kinase activity and its newly found role as an important factor controlling the dynamic relationship between cancer epithelia and stroma position Cav-1 as a new therapeutic target for the treatment of breast cancer.

Autophagy in brain tumors: a new target for therapeutic intervention

The role of autophagy, traditionally considered a cellular homeostatic and recycling mechanism, has expanded dramatically to include an involvement in discrete stages of tumor initiation and development. Gliomas are the most aggressive and also the most common brain malignancies. Current treatment modalities have only a modest effect on patient outcomes. Resistance to apoptosis, a hallmark of most cancers, has driven the search for novel targets in cancer therapy. The autophagy lysosomal pathway is one such target that is being explored in multiple cancers including gliomas and is a promising avenue for further therapeutic development. This review summarizes our current understanding of the autophagic process and its potential utility as a target for glioma therapy.

Established Principles and Emerging Concepts on the Interplay between Mitochondrial Physiology and S-(De)nitrosylation: Implications in Cancer and Neurodegeneration

S-nitrosylation is a posttranslational modification of cysteine residues that has been frequently indicated as potential molecular mechanism governing cell response upon redox unbalance downstream of nitric oxide (over)production. In the last years, increased levels of S-nitrosothiols (SNOs) have been tightly associated with the onset of nitroxidative stress-based pathologies (e.g., cancer and neurodegeneration), conditions in which alterations of mitochondrial homeostasis and activation of cellular processes dependent on it have been reported as well. In this paper we aim at summarizing the current knowledge of mitochondria-related proteins undergoing S-nitrosylation and how this redox modification might impact on mitochondrial functions, whose impairment has been correlated to tumorigenesis and neuronal cell death. In particular, emphasis will be given to the possible, but still neglected implication of denitrosylation reactions in the modulation of mitochondrial SNOs and how they can affect mitochondrion-related cellular process, such as oxidative phosphorylation, mitochondrial dynamics, and mitophagy.

Energy transfer in "parasitic" cancer metabolism: mitochondria are the powerhouse and Achilles' heel of tumor cells

It is now widely recognized that the tumor microenvironment promotes cancer cell growth and metastasis via changes in cytokine secretion and extracellular matrix remodeling. However, the role of tumor stromal cells in providing energy for epithelial cancer cell growth is a newly emerging paradigm. For example, we and others have recently proposed that tumor growth and metastasis is related to an energy imbalance. Host cells produce energy-rich nutrients via catabolism (through autophagy, mitophagy, and aerobic glycolysis), which are then transferred to cancer cells to fuel anabolic tumor growth. Stromal cell-derived L-lactate is taken up by cancer cells and is used for mitochondrial oxidative phosphorylation (OXPHOS) to produce ATP efficiently. However, "parasitic" energy transfer may be a more generalized mechanism in cancer biology than previously appreciated. Two recent papers in Science and Nature Medicine now show that lipolysis in host tissues also fuels tumor growth. These studies demonstrate that free fatty acids produced by host cell lipolysis are re-used via beta-oxidation (beta-OX) in cancer cell mitochondria. Thus, stromal catabolites (such as lactate, ketones, glutamine and free fatty acids) promote tumor growth by acting as high-energy onco-metabolites. As such, host catabolism, via autophagy, mitophagy and lipolysis, may explain the pathogenesis of cancer-associated cachexia and provides exciting new druggable targets for novel therapeutic interventions. Taken together, these findings also suggest that tumor cells promote their own growth and survival by behaving as a "parasitic organism." Hence, we propose the term "Parasitic Cancer Metabolism" to describe this type of metabolic coupling in tumors. Targeting tumor cell mitochondria (OXPHOS and beta-OX) would effectively uncouple tumor cells from their hosts, leading to their acute starvation. In this context, we discuss new evidence that high-energy onco-metabolites (produced by the stroma) can confer drug resistance. Importantly, this metabolic chemo-resistance is reversed by blocking OXPHOS in cancer cell mitochondria with drugs like Metformin, a mitochondrial "poison." In summary, parasitic cancer metabolism is achieved architecturally by dividing tumor tissue into at least two well-defined opposing "metabolic compartments:" catabolic and anabolic.

Hyperactivation of oxidative mitochondrial metabolism in epithelial cancer cells in situ: visualizing the therapeutic effects of metformin in tumor tissue

We have recently proposed a new mechanism for explaining energy transfer in cancer metabolism. In this scenario, cancer cells behave as metabolic parasites, by extracting nutrients from normal host cells, such as fibroblasts, via the secretion of hydrogen peroxide as the initial trigger. Oxidative stress in the tumor microenvironment then leads to autophagy-driven catabolism, mitochondrial dys-function, and aerobic glycolysis. This, in turn, produces high-energy nutrients (such as L-lactate, ketones, and glutamine) that drive the anabolic growth of tumor cells, via oxidative mitochondrial metabolism. A logical prediction of this new "parasitic" cancer model is that tumor-associated fibroblasts should show evidence of mitochondrial dys-function (mitophagy and aerobic glycolysis). In contrast, epithelial cancer cells should increase their oxidative mitochondrial capacity. To further test this hypothesis, here we subjected frozen sections from human breast tumors to a staining procedure that only detects functional mitochondria. This method detects the in situ enzymatic activity of cytochrome C oxidase (COX), also known as Complex IV. Remarkably, cancer cells show an over-abundance of COX activity, while adjacent stromal cells remain essentially negative. Adjacent normal ductal epithelial cells also show little or no COX activity, relative to epithelial cancer cells. Thus, oxidative mitochondrial activity is selectively amplified in cancer cells. Although COX activity staining has never been applied to cancer tissues, it could now be used routinely to distinguish cancer cells from normal cells, and to establish negative margins during cancer surgery. Similar results were obtained with NADH activity staining, which measures Complex I activity, and succinate dehydrogenase (SDH) activity staining, which measures Complex II activity. COX and NADH activities were blocked by electron transport inhibitors, such as Metformin. This has mechanistic and clinical implications for using Metformin as an anti-cancer drug, both for cancer therapy and chemo-prevention. We also immuno-stained human breast cancers for a series of well-established protein biomarkers of metabolism. More specifically, we now show that cancer-associated fibroblasts over-express markers of autophagy (cathepsin B), mitophagy (BNIP3L), and aerobic glycolysis (MCT4). Conversely, epithelial cancer cells show the over-expression of a mitochondrial membrane marker (TOMM20), as well as key components of Complex IV (MT-CO1) and Complex II (SDH-B). We also validated our observations using a bioinformatics approach with data from > 2,000 breast cancer patients, which showed the transcriptional upregulation of mitochondrial oxidative phosphorylation (OXPHOS) in human breast tumors (p < 10(-20)), and a specific association with metastasis. Therefore, upregulation of OXPHOS in epithelial tumor cells is a common feature of human breast cancers. In summary, our data provide the first functional in vivo evidence that epithelial cancer cells perform enhanced mitochondrial oxidative phosphorylation, allowing them to produce high amounts of ATP. Thus, we believe that mitochondria are both the "powerhouse" and "Achilles' heel" of cancer cells.

Cancer: a de-repression of a default survival program common to all cells?: a life-history perspective on the nature of cancer

Cancer viewed as a programmed, evolutionarily conserved life-form, rather than just a random series of disease-causing mutations, answers the rarely asked question of what the cancer cell is for, provides meaning for its otherwise mysterious suite of attributes, and encourages a different type of thinking about treatment. The broad but consistent spectrum of traits, well-recognized in all aggressive cancers, group naturally into three categories: taxonomy ("phylogenation"), atavism ("re-primitivization") and robustness ("adaptive resilience"). The parsimonious explanation is not convergent evolution, but the release of an highly conserved survival program, honed by the exigencies of the Pre-Cambrian, to which the cancer cell seems better adapted; and which is recreated within, and at great cost to, its host. Central to this program is the Warburg Effect, whose malign influence permeates well beyond aerobic glycolysis to include biomass interconversion and genomic heuristics. Warburg-type metabolism and genomic instability are targets whose therapeutic disablement is a major priority.

Caveolin-1 and cancer metabolism in the tumor microenvironment: markers, models, and mechanisms

Caveolins are a family of membrane-bound scaffolding proteins that compartmentalize and negatively regulate signal transduction. Recent studies have implicated a loss of caveolin-1 (Cav-1) expression in the pathogenesis of human cancers. Loss of Cav-1 expression in cancer-associated fibroblasts results in an activated tumor microenvironment, thereby driving early tumor recurrence, metastasis, and poor clinical outcome in breast and prostate cancers. We describe various paracrine signaling mechanism(s) by which the loss of stromal Cav-1 promotes tumor progression, including fibrosis, extracellular matrix remodeling, and the metabolic/catabolic reprogramming of cancer-associated fibroblast, to fuel the growth of adjacent tumor cells. It appears that oxidative stress is the root cause of initiation of the loss of stromal Cav-1 via autophagy, which provides further impetus for the use of antioxidants in anticancer therapy. Finally, we discuss the functional role of Cav-1 in epithelial cancer cells.

Adipocytes promote ovarian cancer metastasis and provide energy for rapid tumor growth

Intra-abdominal tumors, such as ovarian cancer, have a clear predilection for metastasis to the omentum, an organ primarily composed of adipocytes. Currently, it is unclear why tumor cells preferentially home to and proliferate in the omentum, yet omental metastases typically represent the largest tumor in the abdominal cavities of women with ovarian cancer. We show here that primary human omental adipocytes promote homing, migration and invasion of ovarian cancer cells, and that adipokines including interleukin-8 (IL-8) mediate these activities. Adipocyte-ovarian cancer cell coculture led to the direct transfer of lipids from adipocytes to ovarian cancer cells and promoted in vitro and in vivo tumor growth. Furthermore, coculture induced lipolysis in adipocytes and β-oxidation in cancer cells, suggesting adipocytes act as an energy source for the cancer cells. A protein array identified upregulation of fatty acid-binding protein 4 (FABP4, also known as aP2) in omental metastases as compared to primary ovarian tumors, and FABP4 expression was detected in ovarian cancer cells at the adipocyte-tumor cell interface. FABP4 deficiency substantially impaired metastatic tumor growth in mice, indicating that FABP4 has a key role in ovarian cancer metastasis. These data indicate adipocytes provide fatty acids for rapid tumor growth, identifying lipid metabolism and transport as new targets for the treatment of cancers where adipocytes are a major component of the microenvironment.

Autophagy as a mediator of chemotherapy-induced cell death in cancer

Since the 1940s, chemotherapy has been the treatment of choice for metastatic disease. Chemotherapeutic agents target proliferating cells, inducing cell death. For most of the history of chemotherapy, apoptosis was thought to be the only mechanism of drug-induced cell death. More recently, a second type of cell death pathway has emerged: autophagy, also called type II programmed cell death. Autophagy is a tightly regulated process by which selected components of a cell are degraded. It primarily functions as a cell survival adaptive mechanism during stress conditions. However, persistent stress can also promote extensive autophagy, leading to cell death, hence its name. Alterations in the autophagy pathway have been described in cancer cells that suggest a tumor-suppressive function in early tumorigenesis, but a tumor-promoting function in established tumors. Moreover, accumulating data indicate a role for autophagy in chemotherapy-induced cancer cell death. Here, we discuss some of the evidence showing autophagy-dependent cell death induced by anti-neoplastic agents in different cancer models. On the other hand, in some other examples, autophagy dampens treatment efficacy, hence providing a therapeutic target to enhance cancer cell killing. In this paper, we propose a putative mechanism that could reconcile these two opposite observations.

Ketones and lactate increase cancer cell "stemness," driving recurrence, metastasis and poor clinical outcome in breast cancer: achieving personalized medicine via Metabolo-Genomics

Previously, we showed that high-energy metabolites (lactate and ketones) "fuel" tumor growth and experimental metastasis in an in vivo xenograft model, most likely by driving oxidative mitochondrial metabolism in breast cancer cells. To mechanistically understand how these metabolites affect tumor cell behavior, here we used genome-wide transcriptional profiling. Briefly, human breast cancer cells (MCF7) were cultured with lactate or ketones, and then subjected to transcriptional analysis (exon-array). Interestingly, our results show that treatment with these high-energy metabolites increases the transcriptional expression of gene profiles normally associated with "stemness," including genes upregulated in embryonic stem (ES) cells. Similarly, we observe that lactate and ketones promote the growth of bonafide ES cells, providing functional validation. The lactate- and ketone-induced "gene signatures" were able to predict poor clinical outcome (including recurrence and metastasis) in a cohort of human breast cancer patients. Taken together, our results are consistent with the idea that lactate and ketone utilization in cancer cells promotes the "cancer stem cell" phenotype, resulting in significant decreases in patient survival. One possible mechanism by which these high-energy metabolites might induce stemness is by increasing the pool of Acetyl-CoA, leading to increased histone acetylation, and elevated gene expression. Thus, our results mechanistically imply that clinical outcome in breast cancer could simply be determined by epigenetics and energy metabolism, rather than by the accumulation of specific "classical" gene mutations. We also suggest that high-risk cancer patients (identified by the lactate/ketone gene signatures) could be treated with new therapeutics that target oxidative mitochondrial metabolism, such as the anti-oxidant and "mitochondrial poison" metformin. Finally, we propose that this new approach to personalized cancer medicine be termed "Metabolo-Genomics," which incorporates features of both 1) cell metabolism and 2) gene transcriptional profiling. Importantly, this powerful new approach directly links cancer cell metabolism with clinical outcome, and new therapeutic strategies for inhibiting the TCA cycle and mitochondrial oxidative phosphorylation in cancer cells.

Hydrogen peroxide fuels aging, inflammation, cancer metabolism and metastasis: the seed and soil also needs "fertilizer"

In 1889, Dr. Stephen Paget proposed the "seed and soil" hypothesis, which states that cancer cells (the seeds) need the proper microenvironment (the soil) for them to grow, spread and metastasize systemically. In this hypothesis, Dr. Paget rightfully recognized that the tumor microenvironment has an important role to play in cancer progression and metastasis. In this regard, a series of recent studies have elegantly shown that the production of hydrogen peroxide, by both cancer cells and cancer-associated fibroblasts, may provide the necessary "fertilizer," by driving accelerated aging, DNA damage, inflammation and cancer metabolism, in the tumor microenvironment. By secreting hydrogen peroxide, cancer cells and fibroblasts are mimicking the behavior of immune cells (macrophages/neutrophils), driving local and systemic inflammation, via the innate immune response (NFκB). Thus, we should consider using various therapeutic strategies (such as catalase and/or other anti-oxidants) to neutralize the production of cancer-associated hydrogen peroxide, thereby preventing tumor-stroma co-evolution and metastasis. The implications of these findings for overcoming chemo-resistance in cancer cells are also discussed in the context of hydrogen peroxide production and cancer metabolism.

Understanding the metabolic basis of drug resistance: therapeutic induction of the Warburg effect kills cancer cells

Previously, we identified a form of epithelial-stromal metabolic coupling, in which cancer cells induce aerobic glycolysis in adjacent stromal fibroblasts, via oxidative stress, driving autophagy and mitophagy. In turn, these cancer-associated fibroblasts provide recycled nutrients to epithelial cancer cells, "fueling" oxidative mitochondrial metabolism and anabolic growth. An additional consequence is that these glycolytic fibroblasts protect cancer cells against apoptosis, by providing a steady nutrient stream of to mitochondria in cancer cells. Here, we investigated whether these interactions might be the basis of tamoxifen-resistance in ER(+) breast cancer cells. We show that MCF7 cells alone are Tamoxifen-sensitive, but become resistant when co-cultured with hTERT-immortalized human fibroblasts. Next, we searched for a drug combination (Tamoxifen + Dasatinib) that could over-come fibroblast-induced Tamoxifen-resistance. Importantly, we show that this drug combination acutely induces the Warburg effect (aerobic glycolysis) in MCF7 cancer cells, abruptly cutting off their ability to use their fuel supply, effectively killing these cancer cells. Thus, we believe that the Warburg effect in tumor cells is not the "root cause" of cancer, but rather it may provide the necessary clues to preventing chemo-resistance in cancer cells. Finally, we observed that this drug combination (Tamoxifen + Dasatinib) also had a generalized anti-oxidant effect, on both co-cultured fibroblasts and cancer cells alike, potentially reducing tumor-stroma co-evolution. Our results are consistent with the idea that chemo-resistance may be both a metabolic and stromal phenomenon that can be overcome by targeting mitochondrial function in epithelial cancer cells. Thus, simultaneously targeting both (1) the tumor stroma and (2) the epithelial cancer cells, with combination therapies, may be the most successful approach to anti-cancer therapy. This general strategy of combination therapy for overcoming drug resistance could be applicable to many different types of cancer.

Cancer cells metabolically "fertilize" the tumor microenvironment with hydrogen peroxide, driving the Warburg effect: implications for PET imaging of human tumors

Previously, we proposed that cancer cells behave as metabolic parasites, as they use targeted oxidative stress as a "weapon" to extract recycled nutrients from adjacent stromal cells. Oxidative stress in cancer-associated fibroblasts triggers autophagy and  mitophagy, resulting in compartmentalized cellular catabolism, loss of mitochondrial function, and the onset of aerobic glycolysis, in the tumor stroma. As such, cancer-associated fibroblasts produce high-energy nutrients (such as lactate and ketones) that fuel mitochondrial biogenesis, and oxidative metabolism in cancer cells. We have termed this new energy-transfer mechanism the "reverse Warburg effect." To further test the validity of this hypothesis, here we used an in vitro MCF7-fibroblast co-culture system, and quantitatively measured a variety of metabolic parameters by FACS analysis (analogous to laser-capture micro-dissection).  Mitochondrial activity, glucose uptake, and ROS production were measured with highly-sensitive fluorescent probes (MitoTracker, NBD-2-deoxy-glucose, and DCF-DA). Interestingly, using this approach, we directly show that cancer cells initially secrete hydrogen peroxide that then triggers oxidative stress in neighboring fibroblasts. Thus, oxidative stress is contagious (spreads like a virus) and is propagated laterally and vectorially from cancer cells to adjacent fibroblasts. Experimentally, we show that oxidative stress in cancer-associated fibroblasts quantitatively reduces mitochondrial activity, and increases glucose uptake, as the fibroblasts become more dependent on aerobic glycolysis.  Conversely, co-cultured cancer cells show significant increases in mitochondrial activity, and corresponding reductions in both glucose uptake and GLUT1 expression. Pre-treatment of co-cultures with extracellular catalase (an anti-oxidant enzyme that detoxifies hydrogen peroxide) blocks the onset of oxidative stress, and potently induces the death of cancer cells, likely via starvation.  Given that cancer-associated fibroblasts show the largest increases in glucose uptake, we suggest that PET imaging of human tumors, with Fluoro-2-deoxy-D-glucose (F-2-DG), may be specifically detecting the tumor stroma, rather than epithelial cancer cells.

Accelerated aging in the tumor microenvironment: connecting aging, inflammation and cancer metabolism with personalized medicine

Cancer is thought to be a disease associated with aging. Interestingly, normal aging is driven by the production of ROS and mitochondrial oxidative stress, resulting in the cumulative accumulation of DNA damage. Here, we discuss how ROS signaling, NFκB- and HIF1-activation in the tumor microenvironment induces a form of "accelerated aging," which leads to stromal inflammation and changes in cancer cell metabolism. Thus, we present a unified model where aging (ROS), inflammation (NFκB) and cancer metabolism (HIF1), act as co-conspirators to drive autophagy ("self-eating") in the tumor stroma. Then, autophagy in the tumor stroma provides high-energy "fuel" and the necessary chemical building blocks, for accelerated tumor growth and metastasis. Stromal ROS production acts as a "mutagenic motor" and allows cancer cells to buffer-at a distance-exactly how much of a mutagenic stimulus they receive, further driving tumor cell selection and evolution. Surviving cancer cells would be selected for the ability to induce ROS more effectively in stromal fibroblasts, so they could extract more nutrients from the stroma via autophagy. If lethal cancer is a disease of "accelerated host aging" in the tumor stroma, then cancer patients may benefit from therapy with powerful antioxidants. Antioxidant therapy should block the resulting DNA damage, and halt autophagy in the tumor stroma, effectively "cutting off the fuel supply" for cancer cells. These findings have important new implications for personalized cancer medicine, as they link aging, inflammation and cancer metabolism with novel strategies for more effective cancer diagnostics and therapeutics.

Matrix remodeling stimulates stromal autophagy, "fueling" cancer cell mitochondrial metabolism and metastasis

We have previously demonstrated that loss of stromal caveolin-1 (Cav-1) in cancer-associated fibroblasts is a strong and independent predictor of poor clinical outcome in human breast cancer patients. However, the signaling mechanism(s) by which Cav-1 downregulation leads to this tumor-promoting microenvironment are not well understood. To address this issue, we performed an unbiased comparative proteomic analysis of wild-type (WT) and Cav-1(-/-) null mammary stromal fibroblasts (MSFs). Our results show that plasminogen activator inhibitor type 1 and type 2 (PAI-1 and PAI-2) expression is significantly increased in Cav-1(-/-) MSFs. To establish a direct cause-effect relationship, we next generated immortalized human fibroblast lines stably overexpressing either PAI-1 or PAI-2. Importantly, PAI-1/2(+) fibroblasts promote the growth of MDA-MB-231 tumors (a human breast cancer cell line) in a murine xenograft model, without any increases in angiogenesis. Similarly, PAI-1/2(+) fibroblasts stimulate experimental metastasis of MDA-MB-231 cells using an in vivo lung colonization assay. Further mechanistic studies revealed that fibroblasts overexpressing PAI-1 or PAI-2 display increased autophagy ("self-eating") and are sufficient to induce mitochondrial biogenesis/activity in adjacent cancer cells, in co-culture experiments. In xenografts, PAI-1/2(+) fibroblasts significantly reduce the apoptosis of MDA-MB-231 tumor cells. The current study provides further support for the "Autophagic Tumor Stroma Model of Cancer" and identifies a novel "extracellular matrix"-based signaling mechanism, by which a loss of stromal Cav-1 generates a metastatic phenotype. Thus, the secretion and remodeling of extracellular matrix components (such as PAI-1/2) can directly regulate both (1) autophagy in stromal fibroblasts and (2) epithelial tumor cell mitochondrial metabolism.

Evidence for a stromal-epithelial "lactate shuttle" in human tumors: MCT4 is a marker of oxidative stress in cancer-associated fibroblasts

Recently, we proposed a new mechanism for understanding the Warburg effect in cancer metabolism. In this new paradigm, cancer-associated fibroblasts undergo aerobic glycolysis, and extrude lactate to "feed" adjacent cancer cells, which then drives mitochondrial biogenesis and oxidative mitochondrial metabolism in cancer cells. Thus, there is vectorial transport of energy-rich substrates from the fibroblastic tumor stroma to anabolic cancer cells. A prediction of this hypothesis is that cancer-associated fibroblasts should express MCT4, a mono-carboxylate transporter that has been implicated in lactate efflux from glycolytic muscle fibers and astrocytes in the brain. To address this issue, we co-cultured MCF7 breast cancer cells with normal fibroblasts. Interestingly, our results directly show that breast cancer cells specifically induce the expression of MCT4 in cancer-associated fibroblasts; MCF7 cells alone and fibroblasts alone, both failed to express MCT4. We also show that the expression of MCT4 in cancer-associated fibroblasts is due to oxidative stress, and can be prevented by pre-treatment with the anti-oxidant N-acetyl-cysteine. In contrast to our results with MCT4, we see that MCT1, a transporter involved in lactate uptake, is specifically upregulated in MCF7 breast cancer cells when co-cultured with fibroblasts. Virtually identical results were also obtained with primary human breast cancer samples. In human breast cancers, MCT4 selectively labels the tumor stroma, e.g., the cancer-associated fibroblast compartment. Conversely, MCT1 was selectively expressed in the epithelial cancer cells within the same tumors. Functionally, we show that overexpression of MCT4 in fibroblasts protects both MCF7 cancer cells and fibroblasts against cell death, under co-culture conditions. Thus, we provide the first evidence for the existence of a stromal-epithelial lactate shuttle in human tumors, analogous to the lactate shuttles that are essential for the normal physiological function of muscle tissue and brain. These data are consistent with the "reverse Warburg effect," which states that cancer-associated fibroblasts undergo aerobic glycolysis, thereby producing lactate, which is utilized as a metabolic substrate by adjacent cancer cells. In this model, "energy transfer" or "metabolic-coupling" between the tumor stroma and epithelial cancer cells "fuels" tumor growth and metastasis, via oxidative mitochondrial metabolism in anabolic cancer cells. Most importantly, our current findings provide a new rationale and novel strategy for anti-cancer therapies, by employing MCT inhibitors.

Molecular profiling of a lethal tumor microenvironment, as defined by stromal caveolin-1 status in breast cancers

Breast cancer progression and metastasis are driven by complex and reciprocal interactions, between epithelial cancer cells and their surrounding stromal microenvironment. We have previously shown that a loss of stromal Cav-1 expression is associated with an increased risk of early tumor recurrence, metastasis and decreased overall survival. To identify and characterize the signaling pathways that are activated in Cav-1 negative tumor stroma, we performed gene expression profiling using laser microdissected breast cancer-associated stroma. Tumor stroma was laser capture microdissected from 4 cases showing high stromal Cav-1 expression and 7 cases with loss of stromal Cav-1. Briefly, we identified 238 gene transcripts that were upregulated and 232 gene transcripts that were downregulated in the stroma of tumors showing a loss of Cav-1 expression (p ≤ 0.01 and fold-change ≥ 1.5). Gene set enrichment analysis (GSEA) revealed "stemness," inflammation, DNA damage, aging, oxidative stress, hypoxia, autophagy and mitochondrial dysfunction in the tumor stroma of patients lacking stromal Cav-1. Our findings are consistent with the recently proposed "Reverse Warburg Effect" and the "Autophagic Tumor Stroma Model of Cancer Metabolism." In these two complementary models, cancer cells induce oxidative stress in adjacent stromal cells, which then forces these stromal fibroblasts to undergo autophagy/mitophagy and aerobic glycolysis. This, in turn, produces recycled nutrients (lactate, ketones and glutamine) to feed anabolic cancer cells, which are undergoing oxidative mitochondrial metabolism. Our results are also consistent with previous biomarker studies showing that the increased expression of known autophagy markers (such as ATG16L and the cathepsins) in the tumor stroma is specifically associated with metastatic tumor progression and/or poor clinical outcome.

Cytokine production and inflammation drive autophagy in the tumor microenvironment: role of stromal caveolin-1 as a key regulator

Recently, we proposed a new paradigm for understanding the role of the tumor microenvironment in breast cancer onset and progression. In this model, cancer cells induce oxidative stress in adjacent fibroblasts. This, in turn, results in the onset of stromal autophagy, which produces recycled nutrients to "feed" anabolic cancer cells. However, it remains unknown how autophagy in the tumor microenvironment relates to inflammation, another key driver of tumorigenesis. To address this issue, here we employed a well-characterized co-culture system in which cancer cells induce autophagy in adjacent fibroblasts via oxidative stress and NFκB-activation. We show, using this co-culture system, that the same experimental conditions that result in an autophagic microenvironment, also drive in the production of numerous inflammatory mediators (including IL-6, IL-8, IL-10, MIP1a, IFNg, RANTES (CCL5) and GMCSF). Furthermore, we demonstrate that most of these inflammatory mediators are individually sufficient to directly induce the onset of autophagy in fibroblasts. To further validate the in vivo relevance of these findings, we assessed the inflammatory status of Cav-1 (-/-) null mammary fat pads, which are a model of a bonafide autophagic microenvironment. Notably, we show that Cav-1 (-/-) mammary fat pads undergo infiltration with numerous inflammatory cell types, including lymphocytes, T-cells, macrophages and mast cells. Taken together, our results suggest that cytokine production and inflammation are key drivers of autophagy in the tumor microenvironment. These results may explain why a loss of stromal Cav-1 is a powerful predictor of poor clinical outcome in breast cancer patients, as it is a marker of both (1) autophagy and (2) inflammation in the tumor microenvironment. Lastly, hypoxia in fibroblasts was not sufficient to induce the full-blown inflammatory response that we observed during the co-culture of fibroblasts with cancer cells, indicating that key reciprocal interactions between cancer cells and fibroblasts may be required.

Mitochondrial oxidative stress drives tumor progression and metastasis: should we use antioxidants as a key component of cancer treatment and prevention?

The functional role of oxidative stress in cancer pathogenesis has long been a hotly debated topic. A study published this month in BMC Cancer by Goh et al., directly addresses this issue by using a molecular genetic approach, via an established mouse animal model of human breast cancer. More specifically, alleviation of mitochondrial oxidative stress, via transgenic over-expression of catalase (an anti-oxidant enzyme) targeted to mitochondria, was sufficient to lower tumor grade (from high-to-low) and to dramatically reduce metastatic tumor burden by >12-fold. Here, we discuss these new findings and place them in the context of several other recent studies showing that oxidative stress directly contributes to tumor progression and metastasis. These results have important clinical and translational significance, as most current chemo-therapeutic agents and radiation therapy increase oxidative stress, and, therefore, could help drive tumor recurrence and metastasis. Similarly, chemo- and radiation-therapy both increase the risk for developing a secondary malignancy, such as leukemia and/or lymphoma. To effectively reduce mitochondrial oxidative stress, medical oncologists should now re-consider the use of powerful anti-oxidants as a key component of patient therapy and cancer prevention. Please see related research article: http://www.biomedcentral.com/1471-2407/11/191.

The complex interplay between autophagy and NF-κB signaling pathways in cancer cells

Tight regulation of both the NF-κB pathway and the autophagy process is necessary for maintenance of cellular homeostasis. Deregulation of both pathways is frequently observed in cancer cells and is associated with tumorigenesis and tumor cell resistance to cancer therapies. Autophagy is involved in several cellular functions regulated by NF-κB including cell survival, differentiation, senescence, inflammation, and immunity. On a molecular level, autophagy and NF-κB share common upstream signals and regulators and can control each other through positive or negative feedback loops, thus ensuring homeostatic responses. Here, we summarize and discuss the most recent discoveries that shed new light on the complex interplay between autophagy and NF-κB signaling pathways; this certainly has functional relevance in tumorigenesis and tumor responses to therapy.

Antiangiogenic therapy: impact on invasion, disease progression, and metastasis

Antiangiogenic drugs targeting the VEGF pathway have slowed metastatic disease progression in some patients, leading to progression-free survival (PFS) and overall survival benefits compared with controls. However, the results are more modest than predicted by most preclinical testing and benefits in PFS are frequently not accompanied by overall survival improvements. Questions have emerged about the basis of drug resistance and the limitations of predictive preclinical models, and also about whether the nature of disease progression following antiangiogenic therapy is different to classic cytotoxic therapies-in particular whether therapy may lead to more invasive or metastatic behavior. In addition, because of recent clinical trial failures of antiangiogenic therapy in patients with early-stage disease, and the fact that there are hundreds of trials underway in perioperative neoadjuvant and adjuvant settings, there is now greater awareness about the lack of appropriate preclinical testing that preceded these studies. Improved preclinical assessment of all stages of metastatic disease should be a priority for future antiangiogenic drug discovery and development.

Caveolin-1 and mitochondrial SOD2 (MnSOD) function as tumor suppressors in the stromal microenvironment: a new genetically tractable model for human cancer associated fibroblasts

We have recently proposed a new model for understanding tumor metabolism, termed: "The Autophagic Tumor Stroma Model of Cancer Metabolism". In this new paradigm, catabolism (autophagy) in the tumor stroma fuels the anabolic growth of aggressive cancer cells. Mechanistically, tumor cells induce autophagy in adjacent cancer-associated fibroblasts via the loss of caveolin-1 (Cav-1), which is sufficient to promote oxidative stress in stromal fibroblasts. To further test this hypothesis, here we created human Cav-1 deficient immortalized fibroblasts using a targeted sh-RNA knock-down approach. Relative to control fibroblasts, Cav-1 deficient fibroblasts dramatically promoted tumor growth in xenograft assays employing an aggressive human breast cancer cell line, namely MDA-MB-231 cells. Co-injection of Cav-1 deficient fibroblasts, with MDA-MB-231 cells, increased both tumor mass and tumor volume by ~4-fold. Immuno-staining with CD31 indicated that this paracrine tumor promoting effect was clearly independent of angiogenesis. Mechanistically, proteomic analysis of these human Cav-1 deficient fibroblasts identified > 40 protein biomarkers that were upregulated, most of which were associated with i) myofibroblast differentiation, or ii) oxidative stress/hypoxia. In direct support of these findings, the tumor promoting effects of Cav-1 deficient fibroblasts could be functionally suppressed (nearly 2-fold) by the recombinant over-expression of SOD2 (superoxide dismutase 2), a known mitochondrial enzyme that de-activates superoxide, thereby reducing mitochondrial oxidative stress. In contrast, cytoplasmic soluble SOD1 had no effect, further highlighting a specific role for mitochondrial oxidative stress in this process. In summary, here we provide new evidence directly supporting a key role for a loss of stromal Cav-1 expression and oxidative stress in cancer-associated fibroblasts, in promoting tumor growth, which is consistent with "The Autophagic Tumor Stroma Model of Cancer". The human Cav-1 deficient fibroblasts that we have generated are a new genetically tractable model system for identifying other suppressors of the cancer-associated fibroblast phenotype, via a genetic "complementation" approach. This has important implications for understanding the pathogenesis of triple negative and basal breasts cancers, as well as tamoxifen-resistance in ER+ breast cancers, which are all associated with a Cav-1 deficient "lethal" tumor micro-environment, driving poor clinical outcome.

Phosphorylated TP63 induces transcription of RPN13, leading to NOS2 protein degradation

Head and neck squamous cell carcinoma cells exposed to cisplatin display ATM-dependent phosphorylation of the most predominant TP63 isoform (ΔNp63α), leading to its activation as a transcription factor. Here, we found that the phospho-ΔNp63α protein binds to the genomic promoter of RPN13 through the TP63-responsive element. We further found that the phospho-ΔNp63α protein associates with other transcription factors (DDIT3 (also known as CHOP), NF-Y, and NF-κB), activating RPN13 gene transcription. Furthermore, cisplatin-induced and phospho-ΔNp63α-dependent RPN13 gene transcription leads to NOS2 degradation. Finally, we show that RPN13 knockdown by siRNA essentially rescues NOS2 from cisplatin-dependent inactivation. These data provide a novel mechanism for the phospho-ΔNp63α-dependent regulation of NOS2 function in cells upon cisplatin treatment, contributing to the cell death pathway of tumor cells.